activity_scores graph: fix markings initialization
activity_scores/counts: simplify the model class interface

We will need to calculate the average activity for the whole parliament
in an upcoming patch, so make the score_set method avaiable for that.
Also use the occasion to simplify things a bit.
api: add resource for parliament activity scores
activity scores_graph: prepare for multiple data series

We'll add the average activity as an additional data series item in
the next patch, prepare for that here.

No functional change.
flot: add curvedlines plugin

Needed by the next patch.
activity_scores graph: replace all time average with monthly average